Peroxynitrite, the oxidant formed from the diffusion-limited reaction between nitric oxide and superoxide, has been shown to modify low-density lipoprotein (LDL) into a pro-atherogenic form. LDL contains both alpha-tocopherol and gamma-tocopherol. We have shown that peroxynitrite oxidizes alpha-tocopherol to alpha-tocopherol quinone. It has recently been reported by Christian et al. (Proc. Natl. Acad. Sci. USA 943217-3222) that gamma-tocopherol is oxidized to 5-NO2-gamma-tocopherol by peroxynitrite and the gamma-tocopherol may be a more specific scavenger of peroxynitrite than alpha-tocopherol. As both alpha-tocopherol and gamma-tocopherol are present in LDL, it is important to determine which of these reactions is favored. Endogenous levels of both alpha-tocopherol and gamma-tocopherol were monitored when LDL was oxidized in the presence of the peroxynitrite generator, SIN-1. Peroxynitrite generated from SIN-1 reacted preferentially with alpha-tocopherol, spa rin g gamma-tocopherol levels until alpha-tocopherol was depleted. This same sparing effect was demonstrated when dilauryl phosphatidylcholine liposomes containing alpha-tocopherol and gamma-tocopherol were incubated with SIN-1. These data suggest that peroxynitrite reacts preferentially with alpha-tocopherol. We conclude that the reaction between peroxynitrite and gamma-tocopherol is not significant until alpha-tocopherol levels have been depleted.